Apparatus for carrying out a continued union of paper web is known from DE-A-100 43 989 and from U.S. 2001/019757. These documents describe an apparatus for carrying out the union of two paper webs by a mutual compression of the concerned webs, in which the webs are compressed between a pressure roller and an impression roller provided with surface reliefs and/or depressions.
An apparatus usually employed for a continuous union of paper webs comprises, with reference to the outline of FIG. 1, two pairs of rollers and cylinders (A, B; C, D) for embossing paper webs (E, F), a roller (G) for distributing a given amount of glue onto the paper which transits in correspondence of one of the embossing rollers, and an impression roller with rubber-coated surface (H) positioned diametrically opposite to the gluing roller (G): the paper webs (E, F) result embossed as they transit between the surfaces of the corresponding embossing rollers and cylinders, that is, as they pass through the regions indicated by “X” and “Y” in FIG. 1, and become definitively glued by their passing onto the embossing roller (B) and because of the pressure exerted thereon by the rubber-coated roller (H). In FIG. 1, the arrows (VE, VF, VA) indicate the directions of advancement of web (E), web (F) and of the exiting coupled webs (AC).
One drawback relating to this operating technique lies in the fact that, because of the very compliance of the material that sheathes the output pressure roller (H), and of the pressure that this roller exerts on the first embossing cylinder (B), the material of the pressure roller penetrates the surface cavities of the cylinder. As a consequence, a mutual squashing of the two paper webs occurs throughout the space within which the coating material of the pressure roller fits into the cavities of the embossing cylinder (as shown in FIG. 2A) and, accordingly, a kind of re-embossing is produced with a noticeable roughness on one of the two sides of the output material (see FIG. 2B, wherein “L” indicates the deformations in the coupled webs with respect to the ideal profile). Another drawback related to the use of this type of apparatus lies in the fact that the glue distribution's system brings about significant expenses due to the cost of the mechanical members and of the adhesive, and to the difficulty in keeping the system clean.
Another device commonly used for continually joining paper webs comprises a group—downstream of the embossing rollers and cylinders according to the outline of FIG. 3A—consisting of a smooth roller having a hard surface (Z) and a set of small rollers with surface reliefs (W) so disposed that the outputting coupled webs will pass, by resting on the smooth roller (Z), under the small rollers (W). In this way, the reliefs of the small rollers will cause the mutual squashing of the two paper webs and, thereby, their union, without using any adhesive whatsoever.
One drawback related to such a device lies in that, in correspondence of the regions interested by the action of the small rollers, the embossing previously produced is lost and, on the coupled webs exiting from the device, two or more areas developed there along result markedly visible (as illustrated in FIG. 4 where “AC” indicates the coupled webs, “AM” indicate the above said areas, and an array of dots indicates the embossing operated upstream of the group and including the small rollers). In conclusion, also this known operating technique implies a partial but anyway evident lost of the effects derived by the embossing of the two webs which make up the coupled webs. A further drawbacks related to the use of this type of device is that the small rollers tend to vibrate when a given operating speed of the system has been exceeded and, thus, they tend to wear out prematurely, so that an upper limit is to be provided on the operable speed which results incompatible with the current production requirements. The said marks (AM) do not result, more often than not, perfectly centered with respect to the rolls formed when cutting the log of the coupled webs, which contributes to make the same marks (AM) still more unappealing.